Bonded platelet assemblies are used in the manufacture of certain types of components where thin walls with minute and precisely engineered flow passages in complex patterns or where thin walls with complex internal supporting structures are needed. Examples of components of this type are certain heat exchangers, aircraft panels, components for turbine machines, and rocket engines.
The platelets are fabricated from thin sheets of metal, and the assemblies are formed by fusing together a multitude of platelets along their broad faces. The flow passages are formed by through-passages or surface recesses in the individual platelets, with the passages or recesses in adjacent platelets superimposed to achieve the desired flowpath or network of flowpaths through the assembly. These passages or recesses can be formed in a variety of ways, one of the most prominent of which is photochemical machining, or "photoetching." Photoetching to form through-passages which penetrate the thickness of a platelet is commonly termed "through etching" while photoetching to form a recess in one side of the platelet without fully penetrating the platelet is commonly termed "depth etching."
Platelets can be fused together in a variety of ways. Examples are diffusion bonding, roll bonding, and brazing. Diffusion bonding is a particularly effective method, and consists of hot-pressing the platelets together at pressures typically below the yield point of the platelet material and temperatures typically in the range of 50% to 75% of the melting temperature of the platelet material (as measured from room temperature). The etched portions of individual platelets form void regions in the assembly, however, and the bonding pressure will not be transmitted through the assembly at the locations of these void regions. Platelet areas above or below these regions (considering the assembly as a stack of platelets arranged horizontally) will not receive the same pressure as adjacent platelet areas where these voids are not present. The result is nonuniform and perhaps insufficient bonding between platelets. This can cause distortion of the resulting flow passages and deviations from the intended precision of these passages.
Voids in the assembly also interfere with fabrication processes applied to the assembly subsequent to the fusion of the platelets. Many applications and uses of platelet assemblies require the assemblies to be either formed (distorted to a bent or curved shape), machined or welded to other parts. The bending or curving of a platelet assembly can cause buckling of one or more platelets in regions adjacent to voids in an adjacent platelet on the convex side of the bend. In machining operations, the force exerted by a machine tool on platelet regions adjacent to voids will distort those platelets since they lack the support of underlying platelets in those regions. In a welding operation on a region of a platelet assembly above a void, the heat of the welding tool will not dissipate through the assembly at the same rate since the void will not transmit the heat as quickly as the platelet material. The result is overheating and distortion, if not destruction, of the platelet.
Temporary fillers such as wax and certain polymers have been proposed for placement in the void regions after the diffusion bonding. These fillers are removable during processing of the assembly or upon application of heat since they readily melt or volatilize. While these fillers provide some support for smaller voids during forming or machining, they are of little use however for larger voids since they do not prevent bucking or distortion by a machine tool and are not capable of withstanding welding temperatures. In addition, platelet support is not sustained through the bonding process since the filler material melts at the bond temperature.